Energy Extraction from a Galloping PrismPascual E. Camacho1, Daniel W. Carlson2, Yahya Modarres-Sadeghi2

New Mexico State University [1], University of Massachusetts Amherst [2]

IntroductionA body flexibly mounted in flow will experience large oscillations when the frequency of vortex shedding locks-in with the natural frequency of the body. This is called Vortex Induced Vibrations (VIV), which have been widely studied because of the negative effects they can have on structures. In this project, a different approach is taken to understand its effects: the potential for energy extraction from a galloping prism placed in uniform flow. This renewable energy approach can be implemented in offshore wind turbines to power remote sensors that, for example, monitor mooring line vibrations.

Results

AcknowledgementsThis work is supported in part by the National Science Foundation under NSF award number 1460461. Any opinions, findings, conclusions, or recommendations expressed in this material are those of the authors and do not necessarily reflect those of the National Science Foundation.

References[1] Seyed-Aghazadeh, B., Carlson, D., Modarres-Sadeghi, Y., 2017. “Vortex-induced vibration and galloping of prisms with triangular cross-sections,” Journal of Fluid Mechanics, v. 817, p. 590-618. [2] Miller, Harry, 2015. “Energy Extraction from Flow-Induced Vibrations.” [3] Torabi, Corinna, 2017. “Energy Generation from Galloping of a Triangular Prism.”

Experimental DesignGalloping is a high-amplitude, low-frequency instability usually observed in asymmetric structures. Therefore, a triangular prism (Table 1) was used. Tests were performed in a recirculating water tunnel with cross section 1.27 m x 0.5 m x 0.5 m (Figure 2). Figure 1 is the CAD drawing of the prototype used for energy extraction, where the cylinder is attached to low-damping air bearings and translational springs with the cylinder in flow. The improvements made from previous studies are to increase the efficiency in the energy system by reducing losses. The arm, the housing device, and disk were designed such that the rotor disk is mid-cycle for null displacement (Figure 1). Components were 3D printed of solid-fill ABS plastic. Losses at the arm pins were mitigated by rotational bearings.

Figure 1. Energy Extraction Device.

Figure 2. Water Tunnel schematic.

Figure 4. Integral of total power over effective velocity range. Note linearity to swept volume.

A negative linear relation (Figure 4) was found between Dimensionless Amplitude (A/D) and the integral of Power over Reduced Velocity (U*). Previous studies did not limit oscillation amplitude, while in the present work, doing so increases the oscillation frequency. Thus a substantial reduction in generator efficiency can be mitigated by having a higher A/D device work for a much wider effective velocity range, as the swept area of the oscillating cylinder is the prime determinant of the total power extracted over the velocity range.

Natural Frequency (fn)

0.98 Hz

Diameter (D) 1.63 cmWater Flow Velocity (U)

6 - 30 cm/s

Generator Resistance

35 Ω

U* U/(fnD)

Table 1. Experimental parameters.

Conclusions

5 10 15 200

0.5

1

U*

Powe

r (m

W)

5 10 15 200

1

2

3

4

U*

η (%

)

5 10 15 200

1

2

3

4

U*

A/D

5 10 15 200

0.5

1

U*

f os/f n

Prescribed A/D

•- 1.49 ◼- 2.58 ◆- 3.35 ▲- 3.73 ★- 3.96

(d)

(b) (a)

(c)

Figure 3. Reduced Velocity vs (a) Power Generated (b) Efficiency (c) Dimensionless Amplitude, and (d) Frequency Ratio.

Amax/D

Inte

gral

of P

ower

(mW

) ove

r U*

rang

e

Given a fixed radius on the generator disk, the oscillation amplitude builds until it matches the radius prescribed. Note that for smaller prescribed A/D values, it holds the amplitude starting at lower U*. From Figure 3a it can be seen that the lowest Dimensionless Amplitude (A/D) of 1.49 generates the highest power, and from Figure 3b, this set up also leads to the highest efficiencies overall. Marker tones shift to dark once the generator begins DC output, as opposed to AC from partial revolutions. Note that for the lower A/D series, power increases even though A/D does not. This is due to a shift in the oscillation frequency (Figure 3d), yielding more generator cycles per period. The maximum U* was chosen at a point where the arm joint bearings could no longer smoothly handle the impulse of the cylinder reaching a displacement maxima.